Digesters are pressure cooking vessels used to process cellulosic fibrous material, such as wood chips. The digester applies pressure and cooking liquor to process the chips into cellulose pulp from which paper products may be made. A digester vessel is generally a vertical column often having a height greater than 100 feet (about 30 meters). There are essentially two types of continuous digesters: a hydraulic digester and an dual-phase or vapor-phase digester (hereinafter referred to as a vapor-phase digester). A hydraulic digester is a pressure-resistant vessel which is filled with comminuted cellulosic fibrous material and liquid.
The vapor-phase digester includes a pressure vessel that is partially filled with a slurry of the cellulosic fiber and cooking liquor and has a super-atmospheric steam zone (also referred to as the gas zone) above the slurry. The gas in the vapor zone is compressible The pressure within a vapor-phase digester is typically determined by the pressure of the gas at the top of the digester.
Traditionally, wood chips are introduced into hydraulic and vapor phase digesters via mechanical top separators. However, the top separator for a hydraulic digester have typically been different than top separators for vapor-phase digesters. The top separators for both types of digesters have an input port coupled to a feed system. Wood chips, or other comminuted cellulosic fibrous material, are typically fed to continuous digester via the feed system and top separator. The feed system typically includes equipment for de-aerating, heating, pressurizing, and introducing cooking liquor to the chips before transferring a slurry of chips and liquor to the digester. Top separators have conventionally served to introduce chips into a digester and to extract cooking liquor from the input chip slurry. Top separators are used in hydraulic and vapor phase digesters.
In a hydraulic digester, the input slurry of chips and liquor is introduced to a top separator having a downward-directed screw-type conveyor. In the vapor-phase digester, the slurry of chips and liquor is transferred to a screw-type conveyor which moves the slurry upwards so that the chips and liquor overflow the top of the conveyor and fall freely in the steam-filled atmosphere of the digester. This upward flow and overflow of chips and liquid is suited to the vapor-phase digester because it prevents the escape of gas as the slurry is introduced to the digester while providing a weir-type reservoir for removing excess liquid. The top separator for a vapor-phase digester is known in the art as an “inverted top separator”.
In a vapor-phase digester, the wood chips are typically heated by exposure to steam as the chips are introduced to the steam-filled zone at the top of the digester. A vapor-phase digester directly exposes chips to steam for heating by having a chip level that is above the level of the liquid in the digester. The chips are exposed to steam as they are distributed in the vessel by a top separator device and as they rest on the top of the chip pile.
In a vapor phase digester, the chips are “cooked” as they move from the top of the chip pile and progress down into the slurry through the digester vessel. The inverted top separator is mounted in the top section of the digester. The top section of the vessel contains a vapor portion, and the portion of the vessel below the vapor is filled with a liquid-chip slurry.
Top separators for hydraulic and vapor phase digesters remove excess liquid from the slurry. The removed liquid. e.g., liquor, is returned to the feed system (e.g. conventional high pressure feeder) as a source of slurrying liquid. The functions of the top separators for hydraulic and vapor phase digester are similar, but they have distinct applications to their respective type of digester.
The top separator is one of the few major components of a digester vessel that has conventionally had a substantially different construction depending on whether the separator is for a hydraulic or vapor phase digester. See U.S. Pat. Nos. 6,332,954 and 6,325,889. In a hydraulic digester, the top separator has an upper inlet for the chip slurry, a downwardly turning screw conveyor, a bottom chip discharge and the entire separator sits on top of a cone. The cone and the bottom of the separator form a top to the liquid and chip filled portion of the hydraulic digester vessel. In a vapor phase digester, the top separator is mounted directly in the gas filled portion of the vessel (without an underlying cone), has a bottom chip slurry inlet, an upwardly turning screw conveyor, and an annular chip discharge chute at the top of the separator.
Other than the top separator, the major components of the digester, e.g., vessel wall, screens, outlets and liquor piping, are generally common to both types of digesters. There are other differences between the structure of a vapor phase and hydraulic digesters, as is discussed in U.S. Pat. Nos. 6,174,411 and 5,882,477. In view of the differences between the top separators and other differences, vapor phase digesters have in the past not been interchangeable with or convertible to hydraulic digesters, and vice versa. However, there is a long felt need for commonality of components between the digester types and for an ability to convert one type of digester to another.